Re: gdb 8.x - g++ 7.x compatibility

[Martin Sebor <msebor@gmail.com>]

On 02/04/2018 10:06 PM, Simon Marchi wrote:
> Hi Martin,
>
> Thanks for the reply.
>
> On 2018-02-04 02:17 PM, Martin Sebor wrote:
>> Printing the suffix is unhelpful because it leads to unnecessary
>> differences in diagnostics (even in non-template contexts).  For
>> templates with non-type template parameters there is no difference
>> between, say A<1>, A<1U>, A<(unsigned) 1>, or even A<Green> when
>> Green is an enumerator that evaluates to 1, so including the suffix
>> serves no useful purpose.
>
> This is the part I don't understand.  In Roman's example, spelling
> foo<10> and foo<10u> resulted in two different instantiations of the
> template, with different code.  So that means it can make a difference,
> can't it?

Yes, with auto, the type of the constant does determine the type
of the specialization of the template in the source code.

In non-type template arguments, and more to the point I was making,
in diagnostics, the suffix shouldn't or doesn't need to be what
distinguishes the type of the template, even with auto.  The part
"with auto IVAL = 10" in the message

   'void foo<IVAL>::print() [with auto IVAL = 10]':

would be far clearer if auto were replaced by the deduced type,
say along these lines:

   'void foo<IVAL>::print() [with int IVAL = 10]':

rather than relying on the suffix alone to distinguish between
different specializations of the template.  That seems far too
subtle to me.  But I think the diagnostic format is (or should
be) independent of the debug info.

>> In the GCC test suite, it would tend to
>> cause failures due to differences between the underlying type of
>> common typedefs like size_t and ptrdiff_t.  Avoiding these
>> unnecessary differences was the main motivation for the change.
>> Not necessarily just in the GCC test suite but in all setups that
>> process GCC messages.
>
> Ok, I understand.
>
>> I didn't consider the use of auto as a template parameter but
>> I don't think it changes anything.  There, just like in other
>> contexts, what's important is the deduced types and the values
>> of constants, not the minute details of how they are spelled.
>
> Well, it seems like using decltype on a template constant value is
> a way to make the type of constants important, in addition to their
> value.  I know the standard seems to say otherwise (what Manfred
> quoted), but the reality seems different.  I'm not a language expert
> so I can't tell if this is a deficiency in the language or not.
>
>> That said, it wasn't my intention to make things difficult for
>> the debugger.
>
> I hope so :).
>
>> But changing GCC back to include the suffix,
>> even just in the debug info, isn't a solution.  There are other
>> compilers besides GCC that don't emit the suffixes, and there
>> even are some that prepend a cast to the number, so if GDB is
>> to be usable with all these kinds of producers it needs to be
>> able to handle all of these forms.
>
> As I said earlier, there are probably ways to make GDB cope with it.
> The only solution I saw (I'd like to hear about other ones) was to make
> GDB ignore the template part in DW_AT_name and re-build it from the
> DW_TAG_template_* DIEs in the format it expects.  It can already do
> that somewhat, because, as you said, some compilers don't emit
> the template part in DW_AT_name.
>
> Doing so would cause major slowdowns in symbol reading, I've tried it
> for the sake of experimentation/discussion.  I have a patch available
> on the "users/simark/template-suffix" branch in the binutils-gdb
> repo [1].  It works for Roman's example, but running the GDB testsuite
> shows that, of course, the devil is in the details.
>
> Consider something like this:
>
>   template <int *P>
>   struct foo { virtual ~foo() {} };
>
>   int n;
>
>   int main ()
>   {
>     foo<&n> f;
>   }
>
>
> The demangled name that GDB will be looking up is "foo<&n>".  The
> debug info about the template parameter only contains the resulting
> address of n (the value of &n):
>
>  <2><bf>: Abbrev Number: 11 (DW_TAG_template_value_param)
>     <c0>   DW_AT_name        : P
>     <c2>   DW_AT_type        : <0x1ac>
>     <c6>   DW_AT_location    : 10 byte block: 3 34 10 60 0 0 0 0 0 9f   (DW_OP_addr: 601034; DW_OP_stack_value)
>
> I don't see how GDB could reconstruct the "&n" in the template, so
> that's where my idea falls short.

I'm afraid I know too little about the internals of GDB to
fully appreciate the importance of this problem or have
an idea how it could be handled.

With respect to the suffix, I keep coming back to the reality
that even if GCC were to change to emit a format that GDB can
interpret easily and efficiently, there still are other
compilers that emit a different format.  So the conclusion
that a general solution that handles more than just one format
(at least for non-type template arguments without auto) seems
unescapable.

For auto, since it's new, a viable alternative might be to
standardize the debug info format so that eventually all
producers will converge on it. But even that approach won't
help users of existing compilers.

Martin

>
> Simon
>
> [1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/simark/template-suffix
>